(a) It is an affirmative defense that the defendant engaged in the conduct charged to constitute an offense because he was coerced to do so, by the use of or threatened imminent use of, unlawful physical force upon him or a 3d person, which force or threatened force a person of reasonable firmness in his situation would have been unable to resist.

(b) The defense of "duress" as defined in subsection (a) is not available

(1) as to the crime of murder; or

(2) as to any offense when the defendant recklessly places himself in a situation in which it is probable that he will be subjected to the force of threatened force described in subsection (a).